Zero-fired controls are often used to control alternating current to electrical resistance-type heaters. Zero firing applies complete cycles of alternating current, and then blocks complete cycles. Typically, cycles are passed and blocked within one-second time intervals. For example, 40 cycles will be passed and then 20 blocked for 60 Hz service. Zero firing allows the voltage to be blocked and opened as the voltage passes through zero. This avoids opening power at non-zero values, and the resultant spikes in current. These current spikes cause deterioration of heater electrical insulating materials.
A common alternative to zero firing is phase-angle firing. Phase-angle firing applies a portion of each cycle of alternating voltage. Typically, voltage is opened at non-zero values and blocked when the cycle reaches zero.
Devices are available for measuring RMS current, RMS voltage, and true power for phase-angle firing power controls. But these devices are not accurate when full cycles of alternating voltage are passed and blocked. This type of instrumentation would be valuable for trouble-shooting, monitoring and control of electrical equipment and power supplies. Such simple equipment is not available for zero-firing power controls.
Prior art instrumentation to measure RMS current, RMS voltage, or true power for zero-fired applications either have a significant lag time between changes in the measured variable and the indication of the change, or are relatively expensive. Accurately measuring RMS current, RMS voltage, or true power with a measurement that is updated as often as every set of blocked and non-blocked power cycles currently requires computer digitization of the process signals. This requires relatively expensive equipment. It would be advantageous to measure these zero-fired process variables with an apparatus which updates the measurements as frequently as once each cycle of blocked and unblocked alternating current cycles, and which does not require computer digitization of the process signals.
It is therefore an object of the present invention to provide methods and apparatuses capable of measuring true power, RMS voltage, and RMS current for zero-fired electrical power controls. It is a further object to provide such methods and apparatuses which can update the measurement signal as often as each set of blocked and non-blocked alternating current cycles. It is also an object of the present invention to provide such methods and apparatuses in which the process signals are digitized by an inexpensive, dedicated circuit.